Texas, USA 2010 - International Herbage Seed Group

Materials and MethodsBreeding StrategyMass selection was applied to seedling populations derived from „Callide‟ and „Samford‟ Rhodesgrass (2001-06) with the aim of improving both the level of salt tolerance and the agronomicattributes. Based on first generation performance, seedlings selected from „Callide‟ were dividedinto two breeding populations: one comprising leafy, early flowering (less daylength-sensitive)plants with a preference for fine-stemmed types; and the other based on leafy, late floweringplants, rejecting only the very coarse-stemmed types. Selection within the single „Samford‟population focused on leafy, late-flowering, finer-stemmed genotypes.In the first „Callide‟ generation, selection for salt tolerance related only to plant growth andsurvival under high salinity, and was followed by selection for improved agronomic characters.Selection for germination under saline conditions was then added, giving a three-stage selectionprocess for generations 2-4(-5) in „Callide‟ and for generations 1-4 in „Samford‟: (i) germinationunder saline conditions; (ii) growth and survival under saline conditions; and (iii) agronomicperformance under non-saline field conditions.Breeding Procedures2000 „Callide‟ seedlings were grown individually in a peat-vermiculite mix in 50 mm squaretubes. At early tillering, the lower one-third of the tube was immersed in a NaCl solution, and theconcentration progressively raised (0.2 M every 2-3 days) to 0.7 M NaCl, which was maintainedfor c. 2 months during which approximately 85-90% of the seedlings died. The surviving plantswere then established on a spaced-plant grid for field agronomic evaluation.Because salt tolerance for germination and salt tolerance for growth tend to be unrelated,screening for germination under saline conditions was applied to generations 2-4(5) in „Callide‟and generations 1-4 in „Samford‟. Preliminary trials showed that germination was progressivelydelayed (by osmotic effects) and then reduced as the level of salt increased, reaching virtually nilgermination by the 0.3 M NaCl level. At the start of each new generation, seed of the 3 breedingpopulations from the preceding generation was sown and lightly covered by sand in a “floodand-drain”hydroponic system with 0.2 M NaCl plus complete plant nutrients. Despite heavyseeding rates, low numbers of germinated seedlings were obtained due to the heavy selectionpressure exerted for salt tolerance. Following germination, the trays of young seedlings (up to c.1-2 cm tall) growing in sand were placed in salt solutions to c. 3 cm depth. Salt levels wereprogressively increased to 0.7 M NaCl, and the most visibly tolerant seedlings later transferred tothe field as spaced plants.Results and Discussion191